What kind of bonding would you expect to see between these elements?

1. Li and Na

2. K and K

3. Ca and S

4. Mg and Ar

Expert Answers

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The goal of chemical bonding is to fulfill the octet rule. The octet rule states that main group atoms are most content with eight valence electrons (the electrons that are found in their outermost shells/orbits), the electron configuration of noble gasses. Atoms will behave (and form bonds) in a way to obtain this octet. Covalent bonds occur between two nonmetals. During a covalent bond, the atoms involved share their electrons to reach the desired octet. Any electron that is shared goes towards each of the sharing atom's octet. A chemical bond between a metal and a nonmetal is called an ionic bond.  Metals, which have 1 or 2 or 3 valence, will "donate" all their current valence electrons so they drop down to the next lower orbital that is already full. Conversely, nonmetals that have 5 or 6 or 7 valence electrons will take those that are being given by these metals in order to reach their desired octet. Transition metals have what are called delocalized electrons that can "swim" from one transition metal to another in order to fulfill octets. This is called metallic bonding. 

So, I would not expect 1 or 2 from your question to occur as the elements involved are both main group metals. Without being transition metals, delocalization of electrons does not apply. It would be near impossible to obtain an octet using either ionic or covalent bonding. 

3 involves a bond between calcium and sulfur, a metal and nonmetal respectively. Since Ca has 2 electrons and S has 6, a single bond between the two satisfies the octet rule for both atoms. CaS, or calcium sulfide, is the result.

4 also involves a nonmetal and a metal - argon and magnesium. Argon, however, is a noble gas, with a full valence shell of 8 electrons. Because of this, we say it is inert or non-reactive - it has no "motivation" to bond with Mg's 2 valence electrons, and so there will be no bond formation under normal circumstances.

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